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id="article-container"><blockquote>
<p>在介绍 AtomicInteger 时，已经说明在高并发下大量线程去竞争更新同一个原子变量时，因为只有一个线程能够更新成功，其他的线程在竞争失败后，只能一直循环，不断的进行 CAS 尝试，从而浪费了 CPU 资源。而在 JDK 8 中新增了 LongAdder 用来解决高并发下变量的原子操作。下面同样通过阅读源码来了解 LongAdder 。</p>
</blockquote>
<h3 id="介绍"><a href="#介绍" class="headerlink" title="介绍"></a>介绍</h3><p>一个或多个变量共同维持初值为 0 总和。 当跨线程竞争更新时，变量集可以动态增长以减少竞争。 方法 sum 返回当前变量集的总和。</p>
<p>当多个线程更新时，这个类是通常优选 AtomicLong ，比如用于收集统计信息，不用于细粒度同步控制的共同总和。 在低更新竞争，这两个类具有相似的特征。 但在高更新竞争时，使用 LongAdder 性能要高于 AtomicLong，同样要消耗更高的空间为代价。</p>
<img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/longadder-uml.png" align=center />

<p>LongAdder 继承了 Striped64，内部维护一个 Cells 数组，相当于多个 Cell 变量， 每个 Cell 里面都有一个初始值为 0 的 long 型变量。</p>
<h3 id="源码分析"><a href="#源码分析" class="headerlink" title="源码分析"></a>源码分析</h3><h4 id="Cell-类"><a href="#Cell-类" class="headerlink" title="Cell 类"></a>Cell 类</h4><p>Cell 类 是 Striped64 的静态内部类。</p>
<pre><code class="java">@sun.misc.Contended static final class Cell &#123;
    volatile long value;
    Cell(long x) &#123; value = x; &#125;
    final boolean cas(long cmp, long val) &#123;
        return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
    &#125;

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long valueOffset;
    static &#123;
        try &#123;
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class&lt;?&gt; ak = Cell.class;
            valueOffset = UNSAFE.objectFieldOffset
                (ak.getDeclaredField(&quot;value&quot;));
        &#125; catch (Exception e) &#123;
            throw new Error(e);
        &#125;
    &#125;
&#125;</code></pre>
<ol>
<li>Cell 使用 @sun.misc.Contended 注解。</li>
<li>内部维护一个被 volatile 修饰的 long 型 value 。</li>
<li>提供 cas 方法，更新value。</li>
</ol>
<p>其中 @sun.misc.Contended 注解作用是为了减少缓存争用。什么是缓存争用，这里只做下简要介绍。</p>
<blockquote>
<p>伪共享<br>CPU 存在多级缓存，其中最小存储单元是 Cache Line，每个 Cache Line 能存储 64 个字节的数据。<br>在多线程场景下，A B 两个线程数据如果被存储到同一个 Cache Line 上，此时 A B 更新各自的数据，就会发生缓存争用，导致多个线程之间相互牵制，变成了串行程序，降低了并发。<br>@sun.misc.Contended 注解，则可以保证该变量独占一个 Cache Line。<br>详细可参考：<a target="_blank" rel="noopener" href="http://openjdk.java.net/jeps/142">http://openjdk.java.net/jeps/142</a></p>
</blockquote>
<h4 id="Striped64-核心属性"><a href="#Striped64-核心属性" class="headerlink" title="Striped64 核心属性"></a>Striped64 核心属性</h4><pre><code class="java">abstract class Striped64 extends Number &#123;

    /** CPU 的数量，以限制表大小 */
    static final int NCPU = Runtime.getRuntime().availableProcessors();

    /**
     * cell 数组，当非空时，大小是 2 的幂。
     */
    transient volatile Cell[] cells;

    /**
     * Base 值，在无争用时使用，表初始化竞赛期间的后备。使用 CAS 更新 
     */
    transient volatile long base;

    /**
     * 调整大小和创建Cells时自旋锁（通过CAS锁定）使用。
     */
    transient volatile int cellsBusy;
&#125;</code></pre>
<p>Striped64 类主要提供以下几个属性：</p>
<ol>
<li>NCPU：CPU 的数量，以限制表大小。</li>
<li>cells：Cell[] cell 数组，当非空时，大小是 2 的幂。</li>
<li>base：long 型，Base 值，在无争用时使用，表初始化竞赛期间的后备。使用 CAS 更新。</li>
<li>cellsBusy：调整大小和创建Cells时自旋锁（通过CAS锁定）使用。</li>
</ol>
<p>下面看是进入核心逻辑：</p>
<img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/uFCfpw.gif" align=center />


<h4 id="LongAdder-add"><a href="#LongAdder-add" class="headerlink" title="LongAdder#add"></a>LongAdder#add</h4><pre><code class="java">public class LongAdder extends Striped64 implements Serializable &#123;

    public void add(long x) &#123;
        Cell[] as; long b, v; int m; Cell a;
        // cells 是 数组，base 是基础值
        if ((as = cells) != null || !casBase(b = base, b + x)) &#123;
            boolean uncontended = true;
            if (as == null || (m = as.length - 1) &lt; 0 ||
                (a = as[getProbe() &amp; m]) == null ||
                !(uncontended = a.cas(v = a.value, v + x)))
                longAccumulate(x, null, uncontended);
        &#125;
    &#125;
&#125;</code></pre>
<pre><code class="java">
abstract class Striped64 extends Number &#123;
    // 使用 CAS 更新 BASE 的值
    final boolean casBase(long cmp, long val) &#123;
        return UNSAFE.compareAndSwapLong(this, BASE, cmp, val);
    &#125;
    // 返回当前线程的探测值。 由于包装限制，从ThreadLocalRandom复制
    static final int getProbe() &#123;
        return UNSAFE.getInt(Thread.currentThread(), PROBE);
    &#125;
&#125;
</code></pre>
<img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/longadder-add.png" align=center />


<ol>
<li>首先会对 Base 值进行 CAS 更新，当 Base 发生竞争时， 会更新数组内的 Cell 。</li>
<li>数组未初始化，Cell 未初始化， Cell 更新失败，即 Cell 也发生竞争时，会调用 Striped64 的 longAccumulate 方法。</li>
</ol>
<h4 id="Striped64-longAccumulate"><a href="#Striped64-longAccumulate" class="headerlink" title="Striped64#longAccumulate"></a>Striped64#longAccumulate</h4><pre><code class="java">
abstract class Striped64 extends Number &#123;
    /**
     * x 要增加的值
     * wasUncontended 有没有发生竞争 
     */
    final void longAccumulate(long x, LongBinaryOperator fn, boolean wasUncontended) &#123;
        int h;
        // 当前线程有无初始化线程探测值， 给当前线程生成一个 非 0 探测值
        if ((h = getProbe()) == 0) &#123;
            ThreadLocalRandom.current(); // force initialization
            h = getProbe();
            wasUncontended = true;
        &#125;
        boolean collide = false;                // True if last slot nonempty
        // 循环
        for (;;) &#123;
            Cell[] as; Cell a; int n; long v;
            // 数组不为空切数组长度大于 0
            if ((as = cells) != null &amp;&amp; (n = as.length) &gt; 0) &#123;
                // (n - 1) &amp; h 获取到索引，索引处 cell 是否为 null， cell未初始化
                if ((a = as[(n - 1) &amp; h]) == null) &#123;
                    // 判断 cellsBusy 是否为 0
                    if (cellsBusy == 0) &#123;       // Try to attach new Cell
                        Cell r = new Cell(x);   // Optimistically create
                        // cellsBusy == 0 且 使用 casCellsBusy 方法将其更新为 1，失败会继续循环
                        if (cellsBusy == 0 &amp;&amp; casCellsBusy()) &#123;
                            boolean created = false;
                            try &#123;               // Recheck under lock
                                Cell[] rs; int m, j;
                                // 重新检查状态 并创建
                                if ((rs = cells) != null &amp;&amp; (m = rs.length) &gt; 0 &amp;&amp; rs[j = (m - 1) &amp; h] == null) &#123;
                                    rs[j] = r;
                                    created = true;
                                &#125;
                            &#125; finally &#123;
                                // 创建完成之后， 改回 cellsBusy 值
                                cellsBusy = 0;
                            &#125;
                            if (created)
                                break;
                            // 未创建继续循环
                            continue;           // Slot is now non-empty
                        &#125;
                    &#125;
                    collide = false;
                &#125;
                // 传入的 wasUncontended 为 false 即发生碰撞了， 修改为未碰撞， 此处会继续循环，走到下一步，相当于会一直循环这个 cell
                else if (!wasUncontended)       // CAS already known to fail
                    wasUncontended = true;      // Continue after rehash
                // cas 更新 cell 的 value， 成功则返回
                else if (a.cas(v = a.value, ((fn == null) ? v + x : fn.applyAsLong(v, x))))
                    break;
                // 数组到最大长度 即大于等于 CPU 数量， 或者 cells 数组被改变，
                else if (n &gt;= NCPU || cells != as)
                    collide = false;            // At max size or stale
                else if (!collide)
                    collide = true;
                // 乐观锁 进行扩容
                else if (cellsBusy == 0 &amp;&amp; casCellsBusy()) &#123;
                    try &#123;
                        if (cells == as) &#123;      // Expand table unless stale
                            Cell[] rs = new Cell[n &lt;&lt; 1];
                            for (int i = 0; i &lt; n; ++i)
                                rs[i] = as[i];
                            cells = rs;
                        &#125;
                    &#125; finally &#123;
                        cellsBusy = 0;
                    &#125;
                    collide = false;
                    continue;                   // Retry with expanded table
                &#125;
                // 当前探针值不能操作成功，则重新设置一个进行尝试
                h = advanceProbe(h);
            &#125;
            // 没有加 cellsBusy 乐观锁 且 没有初始化，且获得锁成功（此时 cellsBusy == 1）
            else if (cellsBusy == 0 &amp;&amp; cells == as &amp;&amp; casCellsBusy()) &#123;
                boolean init = false;
                try &#123;                           // Initialize table
                    if (cells == as) &#123;
                        Cell[] rs = new Cell[2];
                        rs[h &amp; 1] = new Cell(x);
                        cells = rs;
                        init = true;
                    &#125;
                &#125; finally &#123;
                    cellsBusy = 0;
                &#125;
                if (init)
                    break;
            &#125;
            // 尝试在base上累加
            else if (casBase(v = base, ((fn == null) ? v + x : fn.applyAsLong(v, x))))
                break;                          // Fall back on using base
        &#125;
    &#125;
&#125;</code></pre>
<p>longAccumulate 方法一共有三种情况</p>
<ol>
<li><code>(as = cells) != null &amp;&amp; (n = as.length) &gt; 0</code> 数组不为空且长度大于 0 。<ol>
<li>获取索引处的 cell ， cell 为空则进行初始化。</li>
<li>cell 不为空，使用 cas 更新， 成功 <code>break;</code> 跳出循环， 失败则还在循环内，会一直尝试。</li>
<li>collide 指是否发生冲突，冲突后会进行重试。</li>
<li>冲突后会尝试获得锁并进行扩容，扩容长度为原来的 2 倍，然后继续重试。</li>
<li>获得锁失败（说明其他线程在扩容）会重新进行计算探针值。</li>
</ol>
</li>
<li><code>cellsBusy == 0 &amp;&amp; cells == as &amp;&amp; casCellsBusy()</code> 数组为空，获得乐观锁成功。<ol>
<li>直接初始化数组。</li>
<li>初始数组长度为 2 。</li>
</ol>
</li>
<li><code>casBase(v = base, ((fn == null) ? v + x : fn.applyAsLong(v, x)))</code> 获得乐观锁失败。<ol>
<li>说明有其他线程在初始化数组，直接 CAS 更新 base 。</li>
</ol>
</li>
</ol>
<h4 id="LongAdder-sum"><a href="#LongAdder-sum" class="headerlink" title="LongAdder#sum"></a>LongAdder#sum</h4><pre><code class="java">
public class LongAdder extends Striped64 implements Serializable &#123;

    public long sum() &#123;
        Cell[] as = cells; Cell a;
        long sum = base;
        if (as != null) &#123;
            for (int i = 0; i &lt; as.length; ++i) &#123;
                if ((a = as[i]) != null)
                    sum += a.value;
            &#125;
        &#125;
        return sum;
    &#125;
&#125;</code></pre>
<ol>
<li>数组为空，说明没有发生竞争，直接返回 base 。</li>
<li>数组不为空，说明发生竞争，累加 cell 的 value 和 base 的和进行返回。</li>
</ol>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><h4 id="基本流程"><a href="#基本流程" class="headerlink" title="基本流程"></a>基本流程</h4><ol>
<li>LongAdder 继承了 Striped64，内部维护一个 Cells 数组，相当于多个 Cell 变量， 每个 Cell 里面都有一个初始值为 0 的 long 型变量。</li>
<li>未发生竞争时（Cells 数组未初始化），是对 base 变量进行原子操作。</li>
<li>发生竞争时，每个线程对自己的 Cell 变量的 value 进行原子操作。</li>
</ol>
<h4 id="如何确定哪个线程操作哪个-cell？"><a href="#如何确定哪个线程操作哪个-cell？" class="headerlink" title="如何确定哪个线程操作哪个 cell？"></a>如何确定哪个线程操作哪个 cell？</h4><p>通过 <code>getProbe()</code> 方法获取该线程的探测值，然后和数组长度 <code>n - 1</code> 做 <code>&amp;</code> 操作 (n - 1) &amp; h 。</p>
<pre><code class="java">static final int getProbe() &#123;
    return UNSAFE.getInt(Thread.currentThread(), PROBE);
&#125;</code></pre>
<h4 id="Cells-数组初始化及扩容？"><a href="#Cells-数组初始化及扩容？" class="headerlink" title="Cells 数组初始化及扩容？"></a>Cells 数组初始化及扩容？</h4><p>初始化扩容时会判断 <code>cellsBusy</code>， cellsBusy 使用 <code>volatile</code> 修饰，保证线程见可见性，同时使用 CAS 进行更新。 0 表示空闲，1 表示正在初始化或扩容。</p>
<p>初始化时会创建长度为 2 的 Cell 数组。扩容是创建一个长度是原数组长度 2 倍的新数组，并循环赋值。</p>
<p>如果线程访问分配的 Cell 元素有冲突后，会使用 <code>advanceProbe()</code> 方法重新获取探测值，再次进行尝试。</p>
<h4 id="使用场景"><a href="#使用场景" class="headerlink" title="使用场景"></a>使用场景</h4><p>在高并发情况下，需要相对高的性能，同时数据准确性要求不高，可以考虑使用 LongAdder。</p>
<p>当要保证线程安全，并允许一定的性能损耗时，并对数据准确性要求较高，优先使用 AtomicLong。</p>
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href="#Striped64-%E6%A0%B8%E5%BF%83%E5%B1%9E%E6%80%A7"><span class="toc-text">Striped64 核心属性</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#LongAdder-add"><span class="toc-text">LongAdder#add</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#Striped64-longAccumulate"><span class="toc-text">Striped64#longAccumulate</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#LongAdder-sum"><span class="toc-text">LongAdder#sum</span></a></li></ol></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E6%80%BB%E7%BB%93"><span class="toc-text">总结</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#%E5%9F%BA%E6%9C%AC%E6%B5%81%E7%A8%8B"><span class="toc-text">基本流程</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E5%A6%82%E4%BD%95%E7%A1%AE%E5%AE%9A%E5%93%AA%E4%B8%AA%E7%BA%BF%E7%A8%8B%E6%93%8D%E4%BD%9C%E5%93%AA%E4%B8%AA-cell%EF%BC%9F"><span class="toc-text">如何确定哪个线程操作哪个 cell？</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#Cells-%E6%95%B0%E7%BB%84%E5%88%9D%E5%A7%8B%E5%8C%96%E5%8F%8A%E6%89%A9%E5%AE%B9%EF%BC%9F"><span class="toc-text">Cells 数组初始化及扩容？</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E4%BD%BF%E7%94%A8%E5%9C%BA%E6%99%AF"><span class="toc-text">使用场景</span></a></li></ol></li></ol></div></div><div class="card-widget card-recent-post"><div class="item-headline"><i class="fas fa-history"></i><span>最新文章</span></div><div class="aside-list"><div class="aside-list-item"><a class="thumbnail" href="/2021/09/04/the_converter_converts_front_end_parameters_to_enumerations.html" title="使用 SpringBoot 转换器将前端参数转换为枚举"><img src= "https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/loading.gif" data-lazy-src="https://cdn.jsdelivr.net/gh/liuzhihang/oss/pic/article/uztio4-T5n5Wm.jpg" 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